Preparations having a sustained release property are clinically advantageous in many points in that the time of administration of a drug can be reduced to afford convenience to the patient, the effective concentration of a drug is maintained over a long period of time thereby to effectively keep its pharmaceutical effect, a definite concentration is maintained thereby to reduce toxicity and prevent development of side effects, and the like. In addition, release of a drug in the upper part of the digestive tract is expected for a drug which is desired to directly act in the stomach and in small intestine in the upper part thereof, or for a drug having absorption limited to the upper part of the small intestine. For such a drug, a medical preparation with sustained gastric release has been proposed.
As a technique for preparations with sustained gastric release, there is known a method utilizing selective action at the pylorus by administering, for example, a tablet or capsule having a large diameter (Ogata et al., Pharmaceutical Factory, 3 (9), page 477, 1983). According to this method, however, there are big differences between individuals. Furthermore, residence time may vary depending upon gastric conditions and its contents so that reliability is poor as the form of administering a drug. In addition, the size of the preparation increases difficulty in administration.
As another method, a preparation utilizing hydrophilic colloid (Japanese Patent Application Laid-Open No. 57315/83), a preparation obtained by molding in a hollow shape and coating an active substance on the outer layer (Japanese Patent Application Laid-Open No. 12411/80), a method using foamable microcapsules (Japanese Patent Application Laid-Open No. 76418/77) has been proposed. However, this method is complex and requires extremely severe conditions for production and requires many steps, which are disadvantageous in economical consideration and poorly practicable. In addition, these methods also involve defects that preparations are destroyed by gastric peristaltic motion thereby to lose suspendability and lose the sustained property.
In order to solve the problems described above, there is disclosed a method for producing a medical preparation which is simple and hence highly practical and can retain a drug in the stomach over a long period of time and can release a drug in a constant amount (Japanese Patent Application Laid-Open No. 43108/86). According to this method, a substance capable of forming gel in water, oils and fats which are solid at ambient temperature and a drug are mixed with each other, the mixture is heated to a temperature higher than the melting point of the oils and fats and then cooled to obtain the thus treated product. The product obtained by the method envelops therein many fine closed pores, thereby showing an apparent density of 1 or less. The product has a strength that can sufficiently stand gastric peristaltic motion and by doing so, retains the property of gradually releasing a drug while retaining the drug in the stomach.
In this preparation, however, the entire system of the mixture comprising the gel component, oil and fat component and a drug is solidified by the heating and cooling operations. Therefore, where it is desired to increase the concentration of a drug to a therapeutical level in a short time after administration, rapid initial release of the drug cannot be expected. With respect to release control in the sustained release part, it is possible to control the release to a certain extent by changing a proportion of the gel component and the oil and fat component. However, an excellent prevention effect is exhibited with these components alone and hence, where the content of a drug is extremely small, for example, or where a sparingly soluble drug is used, the preparation might not provide a sufficient release rate corresponding to various properties of drugs.